17:15 〜 19:15
[SEM16-P13] Reorientation of boulders by a tsunami recorded in the remanent magnetization of single silicate crystals
キーワード：別府湾、津波石、ケイ酸塩単結晶、粘性残留磁気
The movement history of boulders is crucial for the reconstruction of paleo-tsunamis; however, the reworking movements of the boulders remain unresolved. Viscous remanent magnetization (VRM) of boulders has been used to reconstruct the multiple geologic events that result in significant movements of a rock. If a magnetic-mineral bearing rock is reoriented in the geomagnetic field, the magnetism of the smaller magnetic grains aligns to the field direction with time. VRM investigations of boulders reworked by tsunami events provide information about important constraints on the frequency of the events.
There are andesite boulders on Itogahama beach along the north coast of Beppu Bay. The mineral composition of the boulders is consistent with the Akisada pyroclastic flow deposit (0.43 Ma) that crops out just behind of the tidal flat, indicating that the boulders were probably fell from the outcrops by erosion. The maximum boulder weight decreases seaward, suggesting that some of the boulders may have been reworked seaward by backwash flows of past tsunami(s) after being fallen from the outcrops. Historical records show that the tsunami height associated with the 1596 CE Keicho-Bungo earthquake (M=7.0±1/4) was 4–5 m around the study site. Moreover, Yamada et al. (2021) identified four tsunami deposits during the past 7,300 year sedimentary sequence at a coastal marsh in south coast of the bay. Therefore, we conducted paleomagnetic analysis to reveal the boulders as being of tsunami origin. Although the vector plots of one-inch core samples have shown the two-magnetic components, the demagnetization temperatures of softer magnetic components were higher than 200°C, which indicates the alteration of pre-existing magnetic minerals. To avoid such difficulties, we use the remanent magnetization of single phenocryst mineral (plagioclase and pyroxene). The result has shown the multiple magnetic components and the inflection points are lower than 200°C. The low temperature component suggests the reorientation of the boulder by the tsunami. Moreover, to obtain the large dataset of thermal demagnetization form the single crystals, we embed 99 crystals in pits drilled into a glass slide and are planning to apply the measurement of SQUID imaging system.
There are andesite boulders on Itogahama beach along the north coast of Beppu Bay. The mineral composition of the boulders is consistent with the Akisada pyroclastic flow deposit (0.43 Ma) that crops out just behind of the tidal flat, indicating that the boulders were probably fell from the outcrops by erosion. The maximum boulder weight decreases seaward, suggesting that some of the boulders may have been reworked seaward by backwash flows of past tsunami(s) after being fallen from the outcrops. Historical records show that the tsunami height associated with the 1596 CE Keicho-Bungo earthquake (M=7.0±1/4) was 4–5 m around the study site. Moreover, Yamada et al. (2021) identified four tsunami deposits during the past 7,300 year sedimentary sequence at a coastal marsh in south coast of the bay. Therefore, we conducted paleomagnetic analysis to reveal the boulders as being of tsunami origin. Although the vector plots of one-inch core samples have shown the two-magnetic components, the demagnetization temperatures of softer magnetic components were higher than 200°C, which indicates the alteration of pre-existing magnetic minerals. To avoid such difficulties, we use the remanent magnetization of single phenocryst mineral (plagioclase and pyroxene). The result has shown the multiple magnetic components and the inflection points are lower than 200°C. The low temperature component suggests the reorientation of the boulder by the tsunami. Moreover, to obtain the large dataset of thermal demagnetization form the single crystals, we embed 99 crystals in pits drilled into a glass slide and are planning to apply the measurement of SQUID imaging system.